1. Field of the Invention
The present invention relates to a method and apparatus for controlling a ventilator such as a mechanical ventilator (i.e. an artificial respirator) or a respiratory assist device. In particular, the present invention relates to a method and apparatus for controlling a ventilator based on the measured levels of oxygen of the patient on the ventilator, as well as other physical conditions of the patient.
2. Background of the Invention
Mechanical ventilators and other respiratory assist devices are extensively used to treat and manage all patient populations. In the past few decades, there have been significant changes in the features offered by the ventilators and they have become increasingly responsive to individual patient needs. However, despite much advancement in these devices, most ventilators used today are still mainly open-loop controlled devices and their added features have to some extent contributed to their complexity. The clinicians are required to make many important selections among the wide range of options available in advanced mechanical ventilators. Optimal adjustment of these machines oftentimes requires in depth knowledge about the ventilator along-with thorough review of the patient's status and his/her underlying illness. These adjustments are particularly cumbersome and frequent in more fragile and less medically stable patients.
There have been many attempts in the past to automatically control some of the main outputs of mechanical ventilators. See Y. Mitamura et al., “A dual control system for assisting respiration,” Medical and Biological Engineering, vol. 13, no. 6, pages 846-854, 1975, Yu et al., “Improvement in arterial oxygen control using multiple model adaptive control procedures,” IEEE Transactions on Biomedical Engineering, BME-34(8), pages 567-574, 1987, and U.S. Pat. No. 4,986,268 to F. T. Tehrani, issued Jan. 22, 1991, entitled “Method and apparatus for controlling an artificial respirator.”
Also, see U.S. Pat. No. 5,103,814 to T. Maher, issued Apr. 14, 1992, entitled “Self-compensating patient respirator,” Morozoff P. E., and Evans R. W., “Closed-loop control of SaO2 in the neonate,” Biomedical Instrumentation and Technology, vol. 26, pages 117-123, 1992, U.S. Pat. No. 5,365,922 to D. B. Raemer issued Nov. 22, 1994 entitled “Closed-loop non-invasive oxygen saturation control system,” Tehrani et al. “Closed-loop control of the inspired fraction of oxygen in mechanical ventilation,” Journal of Clinical Monitoring and Computing, vol. 17, No. 6, pages 367-376, 2002, and U.S. Pat. No. 6,671,529 to N. R. Claure et al., issued Dec. 30, 2003, entitled “System and method for closed-loop controlled inspired oxygen concentration.”
Some of the prior art on this subject is focused on controlling the patient's oxygenation, and some is intended to automatically control the breathing frequency and tidal volume. The systems intended for controlling only the oxygen level of the patient on the ventilator, either do not provide the automation of all factors that affect oxygenation and/or they do not provide a reliable and sufficiently robust response against oxygen disturbances.
In addition to advancement in mechanical ventilators, there have been many attempts in recent years to prevent the collapse of the airways and apnea in spontaneously breathing patients specially during sleep, by using less elaborate machines than mechanical ventilators, generally known as CPAP machines (CPAP stands for Continuous Positive Airway Pressure). In these machines, either a constant pressure is applied to the patient's airways throughout respiration (i.e. CPAP), or a combination of CPAP and pressure support in inspiration is used to ventilate the patient (e.g. bilevel CPAP machines). See U.S. Pat. No. 4,773,411 to J. B. Downs issued Sep. 27, 1988, entitled “Method and apparatus for ventilatory therapy,” International Patent Publication No. WO 99/61088 to Resmed Limited, issued Dec. 2, 1999, entitled “Ventilatory assistance for treatment of cardiac failure and Cheyne-Stokes breathing,” U.S. Pat. No. 6,539,940 to R. J. Zdrojkowski et al., issued Apr. 1, 2003, entitled “Breathing gas delivery method and apparatus,” and U.S. Pat. No. 6,752,151 to P. D. Hill, issued Jun. 22, 2004, entitled “Method and apparatus for providing variable positive airway pressure.”
In one embodiment, the present invention describes a method and apparatus that can reliably and robustly control PEEP (or CPAP), and FIO2. These are novel features which significantly improve the oxygenation of patients during ventilatory therapy provided by mechanical ventilators as well as respiratory devices such as CPAP machines.
Furthermore, in a more elaborate embodiment of the invention, in addition to PEEP (or CPAP) and FIO2, the I:E ratio of the patient can be automatically adjusted and by further inclusion of the features of U.S. Pat. No. 4,986,268, the breathing frequency, and tidal volume can be automatically controlled in mechanical ventilation. Application of these features results in a significantly more effective and optimal treatment to the patient based on his/her conditions and requirements, in total or assist ventilatory therapy.